Many coals contain from about 15 to about 40 weight percent of moisture that not only does not add to the fuel value of the coal, but also increases the transport cost of the coal. Several United States patents have issued to the applicant for drying coal in a fluidized bed reactor. These include U.S. Pat. Nos. 5,830,246, entitled “Process for Processing Coal”, 5,830,247 (the '247 patent), entitled “Process for Processing Coal”, 5,858,035, entitled “Process for Processing Coal”, 5,904,741, entitled “Process for Processing Coal”, 6,162,265, (the '265 patent) entitled “Process for Processing Coal” and 7,537,622, entitled “Process for Drying Coal”. The entire disclosure of each of these United States patents is hereby incorporated by reference into this specification.
The '265 patent is illustrative of these patents that teaches a process for preparing an irreversibly dried coal with a first fluidized bed reactor that includes a first fluidized bed with a fluidized bed density of from about 30 to about 50 pounds per cubic foot is maintained at a temperature of from about 480 to about 600 degrees Fahrenheit. Coal with a moisture content of about 15 to about 30 percent and a particle size such that all of the coal particles in such coal are in the range of from 0 to 2 inches is fed into the first fluidized bed. In addition, liquid phase water, inert gas, and air is fed into the first fluidized bed that is subjected, along with the coal, to a temperature of about 480 to about 600 degrees Fahrenheit for about 1 to about 5 minutes while simultaneously comminuting and dewatering said coal. This process produces coal of coarse fraction and a fine fraction. The fine fraction is sent to a cyclone wherein a portion of the fine fraction is removed and fed to a cooler that reduces the temperature of the fine fraction by at least about 300 degrees Fahrenheit. The comminuted and dewatered coal is then passed to a second fluidized bed reactor that includes a second fluidized bed with a fluidized bed density of about 30 to about 50 pounds per cubic foot and a temperature of about 215 to about 250 degrees Fahrenheit. Water, inert gas and oil of about 0.5 to about 3.0 weight percent of mineral oil having an initial boiling point of at least about 900 degrees Fahrenheit are also fed to the second fluidized bed. The temperature change experienced by the coal as it moves from the first fluidized bed reactor to the second fluidized bed reactor, from about 480 to about 600 degrees Fahrenheit to about 215 to about 250 degrees Fahrenheit, and associated processes occur in less than about 120 seconds.
The process described in the '265 patent works well with reactors with a diameter of less than about 4 feet, which generally have an output of about 200 tons per day. With larger reactors, wherein the output(s) often exceed 1,000 tons per day, the process is often not as efficient. Without wishing to be bound to any particular theory, it is believed that, as the size of the reactor increases, the gas velocity produced in the process increases geometrically, often to the point where the desired density of the fluidized bed used suffers. As the density of the fluidized bed declines, the efficiency of the drying process decreases.
It is thus a long felt need to provide an improved process for drying coal that is efficient with larger fluidized bed reactors.